1. Field of the Invention
This invention relates to rocket engines. More specifically, the invention is dual expander cycle type of rocket engine that incorporates a closed-cycle heat exchanging unit as a means to safely, efficiently and reliably drive the engine's turbomachinery.
2. Description of the Related Art
A well known upper stage rocket engine is the expander cycle rocket engine that runs on a fuel/oxidizer propellant combination. The expander cycle is based upon the concept that the engine's turbomachinery are driven with gases warmed through regenerative cooling of the engine's thrust chamber assembly. The expander cycle engine eliminates the need for secondary combustion zones such as gas generators or pre-burners.
The propellant combination most suitable for upper-stage, expander cycle rocket engine use is the hydrogen/oxygen combination due to its high specific impulse. Unfortunately, there are large differences in fluid densities when these propellants are used in their cryogenic forms as is generally the case. Typically, the differing densities necessitate the use of centrifugal pumps that must be run at substantially different speeds. The different pump speeds are achieved by using a single turbopump unit and a mechanical gearbox arrangement that allows the single unit to operate two pump shafts at two different speeds. However, the mechanical nature of a gearbox makes the single turbopump/gearbox arrangement susceptible to reliability issues. Thus, the signal turbopump/gearbox embodiment of the expander cycle rocket engine is not an optimal design option.
Another approach to the differing densities problem associated with the hydrogen/oxygen propellant combination is to use two turbopump units with each unit operating at a different speed. In the most typical design, heated hydrogen gas is used to drive both of the turbopumps. However, within the oxygen turbopump, a complicated purged seal must be provided to keep the oxygen and hydrogen separated from one another to thereby avoid a catastrophic failure/explosion. Thus, this approach is inherently risky and is also not an optimal design option.
Still another approach to the differing densities problem associated with the hydrogen/oxygen propellant combination is to use what is called a dual expander cycle engine. In this design, warmed hydrogen gas is used to drive the hydrogen turbopump and warmed oxygen is used to drive the oxidizer turbopump. The fundamental drawback of this embodiment of the expander cycle is that separate thrust chamber cooling circuits (i.e., one for hydrogen and one for oxygen) are necessary in order to generate the necessary turbopump drive gases. Unfortunately, he inclusion of two separate cooling circuits is difficult to incorporate into an actual rocket system duet to weight considerations. Further, there is inherent risk in using oxygen as a coolant since there is potential for catastrophic failure resulting from coolant jacket leaks. For these reasons, this particular embodiment of the expander cycle engine is not an optimal design option.